/Og   (Global Optimizations)

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Feature Only in Professional and Enterprise Editions   Code optimization is supported only in Visual C++ Professional and Enterprise Editions. For more information, see .

This option provides local and global optimizations, automatic-register allocation, and loop optimization. (To find this option in the development environment, click Settings on the Project menu. Then click the C/C++ tab, and click Optimizations in the Category box. Under Optimizations, click Global Optimizations.)

• Local and global common subexpression elimination

  In this optimization, the value of a common subexpression is calculated once. In the following example, if the values of b and c do not change between the three expressions, the compiler can assign the calculation ofb + cto a temporary variable, and substitute the variable for b + c:

  a = b + c;
  d = b + c;
  e = b + c;
  

  For local common subexpression optimization, the compiler examines short sections of code for common subexpressions. For global common subexpression optimization, the compiler searches entire functions for common subexpressions.

• Automatic register allocation

  This optimization allows the compiler to store frequently used variables and subexpressions in registers; the register keyword is ignored.

• Loop optimization

  This optimization removes invariant subexpressions from the body of a loop. An optimal loop contains only expressions whose values change through each execution of the loop. In the following example, the expressionx + ydoes not change in the loop body:

  i = -100;
  while( i < 0 )
  {
      i += x + y;
  }
  

  After optimization,x + yis calculated once rather than every time the loop is executed:

  i = -100;
  t = x + y;
  while( i < 0 )
  {
      i += t;
  }
  

  Loop optimization is much more effective when the compiler can assume no aliasing, which you set with Assume No Aliasing (/Oa) or Assume Aliasing Across Function Calls (/Ow).

Note   You can enable or disable global optimization on a function-by-function basis using the optimize pragma with the g option.

For related information, see Generate Intrinsic Functions (/Oi), Improve Float Consistency (/Op), and Full Optimization (/Ox).